The present invention relates to the structure of an optical fiber for optical communication.
An optical fiber is the communication line in which an optical signal is transmitted through the total reflection of optical energy. It is composed of a cladding of uniform refractive index and a core with larger refractive index than that of the cladding. An optical fiber whose refractive index changes stepwise at the core-cladding interface as shown in FIG. 1, is called a step index optical fiber. An optical fiber whose index changes smoothly in the core regions as shown in FIG. 2, is called a graded index optical fiber. In FIGS. 1 and 2, the abscissa shows the radial length from the core center, and the ordinate shows a refractive index.
In FIG. 2, the refractive index (n) in the core region satisfies, EQU n=n.sub.0 {1-(r/a).sup..alpha. }, (1)
where (n.sub.0) is the refractive index at the core axis, (r) is the radial length from the core center, (a) is the core radius, .alpha. is a power exponent, and .DELTA. is the relative index difference between core and cladding. Here, .DELTA. is defined by EQU .DELTA.=(n.sub.0 -n.sub.e) n.sub.0, (2)
where n.sub.e is the refractive index of the cladding, and it should be appreciated that the refractive index of the cladding is constant throughout the entire cladding region.
It has been well known that there are two kinds of optical fibers which are classified according to the propagating mode nunber in the fiber. One is called a single mode optical fiber in which only a single mode (LP.sub.01 mode) propagates. Although a single mode fiber has the advantage of broad bandwidth (for instance, the bandwidth is larger than 30 GH.sub.z.Km), it has the disadvantage that the connection or splicing of fibers is very difficult since the core diameter is very small (for instance, the core diameter is several .mu.m). The other type of optical fiber is called a multimode optical fiber in which a plurality of modes propagate in the fiber. The multimode optical fiber has the advantage of large core diameter (for instance, several tens .mu.m). However, a multimode optical fiber has the disadvantage of narrow bandwidth since the group delays of modes are different. In discussing the characteristics of an optical fiber, the normalized frequencly (v) defined by Eq. (3) is well utilized. ##EQU2## where .lambda. is the wavelength, and the condition .DELTA.&lt;1 must be satisfied.
Theoretically speaking, an optical fiber having the value (v) smaller than 2.405 operates as a single mode optical fiber, and that having the value (v) equal to or larger than 2.405 operates as a multimode optical fiber, as far as a step index optical fiber is concerned. For a single mode optical fiber with a given v, a possible means to get a larger core radius (a) is to decrease the value of .DELTA.. Said means is derived from Eq. (3). Although the above means has been tried, the total performance of the resultant optical fiber is not satisfactory because a new undesirable disadvantage such as an increase of bending loss is generated. On the other hand, a multimode optical fiber can have a value (v) larger than several tens, but as mentioned above, the bandwidth of a multimode optical fiber is narrow. Although the value of .alpha. is designed to be approximately 2 in order to obtain a wider bandwidth in a graded index optical fiber, the bandwidth thus obtained is still unsatisfactory.